1. Field of the Invention
This invention relates to a force sensing system. It may be used as a device to measure rowing performance; it enables the force transmitted by a rower through an oar to an oarlock to be measured. The oarlock is the rotating support for the oar.
2. Description of the Prior Art
In the sport of competitive rowing, the analysis of a rower's technique and its contribution to boat speed is greatly enhanced by the measurement of physical parameters such as the forces acting on the oars and hull. One such measurement is the force applied by an oar to its oarlock. FIG. 1 shows a plan view of a typical section of rowing boat, including recognisable features such as the sliding seat (1), footplate (2), and an oar (3). The oarlock (4) is a support for the oar that rotates about a fixed near-vertical pin (5) situated on an outrigger (6). Whilst the interaction of forces and inertia on a rowing boat are complex, the force between oar and oarlock provides a straight-forward representation of the forces on the oar, and moreover offers a convenient location to insert some instrumentation.
A number of instrumented oarlocks have been previously devised. An example made by K. Philter of East Germany in the 1950s transcribed the deflection of a sprung component in the oarlock to a paper record.
An electro-mechanical system used by the University of Pennsylvania in the 1960s consisted of an oarlock mounted on a sliding spring loaded mechanism. The displacement of the oarlock during the stroke was proportional to the applied force, and this was converted by means of switches into an electrical signal for display to the coxswain. Practical disadvantages reportedly included the difficulty of moving the system from one boat to another, and the vulnerability of the mechanism to damage.
More recently, strain gauges have been used to measure the loads on the oarlock or pin, with the resultant signals captured by electronic data acquisition systems for display on the boat or relayed to the coach.
An example of this was developed by V. Kleshnev of Russia in the 1990s (see SU 1650171), and consists of a standard oarlock modified to include a load cell. The whole mechanism rotates about a standard pin, and provides a force measurement perpendicular to the oar. To resolve the force into a direction parallel to the hull (i.e. the effective propulsive force) it is necessary to measure the angle of the oarlock relative to the hull. The device enables such a measurement, using a gear mechanism and potentiometer. However, the device is disadvantaged by its size, which would preclude its use on most modern rigs without modification. The rotation of the load cell also presents a problem, as the flexible cable required to transmit the signals to the hull would provide a mechanical weak-spot.
Another approach has the oarlock rotating on a second pin which is parallel and located sternwards of the boat's standard pin. The two pins are connected using a load cell through which all forces are transmitted. Whilst this results in a force measurement parallel to the hull, the method inherently changes the geometry of the boat, so cannot be considered to be provide ‘non-invasive’ testing.
Other systems measure the force transmitted through the oats, typically by modifying them to incorporate a strain sensing element. This technique is quite common, as the modifications are relatively easy to carry out. However, regular recalibration is usually required, as the readings will be proportional to the elasticity of the oar which, given the materials commonly used in oar manufacture, typically varies widely with temperature and age. Other drawbacks include the vulnerability of the cabling required to take the signals off the oar, and the requirement to supplement the force measurement with the angle of the oar in order to resolve the effective propulsive force.
An assessment of the rower's technique can also be derived from a measurement of the reactive force between the athlete's feet and the hull. This measurement effectively superimposes the forces due to the inertial effects of the athlete's body movement and the forces applied to the oar handle. Whilst providing a fairly complete picture of the effect of the rower on the hull, the inability to separate out the force on the oar means that it is an incomplete solution for analysing rowing technique. There are also obstacles to achieving any degree of accuracy: the direction and point of application of force has wide variability; the athlete's feet may tend to contact with the sides of the hull in a narrow boat, thereby channelling some of the applied force away from the sensing element.
The measurement of oar or oarlock angle is also an important feature of measuring rowing performance since (in addition to allowing a force to be resolved in a direction parallel to the hull, as in Kleshnev SU 1650171), it allows one to measure force as a function of oar angle, plus the total arc of the rowing stroke. Angle measurement is typically implemented by means of a potentiometer whose body is fixed relative to the hull and whose rotor is mechanically linked to the oar or oarlock by gearing, lever arms or a tensioning band. Such methods (e.g. as implemented by Kleshnev—see above) usually requires several steps for installation and expensive sensors to achieve the required ruggedness.
Despite the valuable data they yield, prior art systems have yet to achieve wide-spread usage and are confined mainly to the higher echelons of the sport. This reflects the shortcomings of the prior art, which tend to be deficient in at least one aspect of reliability, ease of installation, ease of use, accuracy of measurement and affordability.
All prior art systems require modification of the rig of the boat, some have compromised accuracy, and most are inherently fragile or vulnerable. These shortcomings help to explain why such sensors have failed to achieve any widespread use, despite the obvious advantages of the data they can provide.